Microplastic Pollution and Catchment Morphometry in First-Order Streams of the Ona River System, Southwestern Nigeria.

Microplastic (MP) pollution in river systems is increasingly recognized as a critical environmental challenge, yet the extent to which basin-scale geomorphology controls MP distribution remains poorly constrained, particularly in rapidly urbanizing tropical catchments. This study integrates catchment morphometric analysis with water-column MP sampling across eight first-order streams in the Ona River Basin, southwestern Nigeria. MP concentrations exhibited pronounced spatial heterogeneity (13.58-63.58 items L⁻¹), with consistently higher values in streams draining densely populated and commercially active areas. Simple and multiple regression analyses revealed weak and statistically non-significant relationships between MP concentrations and classical morphometric indices, including stream length, form factor, circularity ratio, and elongation ratio, although the latter showed a marginal inverse trend. Diagnostic evaluation confirmed limited predictive capacity of morphometric parameters. These exploratory results (120 data points across 8 streams) suggest localized anthropogenic inputs are more closely associated with MP distribution than basin-scale geomorphic form, though anthropogenic and connectivity variables were assessed qualitatively rather than measured directly. The findings highlight the limits of applying traditional morphometric frameworks to synthetic pollutants in urban tropical rivers and the need for integrated approaches combining geomorphological context with socio-environmental indicators.